The development of a rapid, simple method for carrying out a range of biochemical assays would greatly enhance the field of diagnostics, particularly in areas such as the health care, environmental monitoring and food industries. For effective "on-site" use, the device should be operated with the minimum amount of manual manipulation and be suitable for use by non-specialists operators. Self-contained disposable test cards utilizing liquid flow channels are a convenient and economical assay platform and are used for detecting or quantitating biological analytes based on analyte-specific binding between an analyte and an analyte-binding receptor or agent. Analyte/analyte binding pairs encountered commonly in diagnostics include antigen-antibody, hormone-receptor, drug-receptor, cell surface antigen-lectin, biotin-avidin, and complementary nucleic acid strands. The advantages of a card format include convenience, privacy, and low cost. A variety of methods for detecting analyte-binding agent interactions have been developed. The simplest of these is a solid-phase format employing a reporter labeled analyte-binding agent whose binding to or release from a solid surface is dependent on the presence of analyte. In a typical solid-phase sandwich type assay, for example, the analyte to be measured is an analyte with two or more binding sites, allowing analyte binding both to a receptor, e.g., antibody, carried on a solid surface, and to a reporter-labeled second receptor. The presence of analyte is detected (or quantitated) by the presence (or amount) of reporter bound to solid surface. In a typical solid-phase competitive binding assay, an analyte competes with a reporter-labeled analyte analog for binding to a receptor (analyte-binding agent) carried on a solid support. The amount of reporter signal associated with the solid support is inversely proportional to the amount of sample analyte to be detected or determined. The reporter label used in both solid-phase formats is typically a visibly detectable particle or an enzyme capable of converting a substrate to an easily detectable product. Simple spectrophotometric devices allow for the quantitation of the amount of reporter label, for quantifying amount of analyte.
There is increasing interest in developing electrochemical biosensors capable of detecting and quantifying analyte-receptor binding events. A biosensor is defined as being a unique combination of a receptor for molecular recognition, for example a selective layer with immobilized antibodies, and a transducer for transmitting the interaction information to processable signals. Biosensors offer an alternative format for the performance of established immunoassays.
One general type of biosensor employs an electrode surface in combination with current or impedance measuring elements for detecting a change in current or impedance in response to the presence of a ligand-receptor binding event. This type of biosensor is disclosed, for example, in U.S. Pat. No. 5,567,301.
Biosensors based on surface plasmon resonance (SPR) effects have also been proposed, for example, in U.S. Pat. No. 5,485,277. These devices exploit the shift in SPR surface reflection angle that occurs with perturbations, e.g., binding events, at the SPR interface. Finally, a variety of biosensors that utilize changes in optical properties at a biosensor surface are known, e.g., U.S. Pat. No. 5,268,305.
The interest in biosensors is spurred by a number of potential advantages over strictly biochemical assay formats. First, electrochemical biosensors may be produced, using conventional microchip technology, in highly reproducible and miniaturized form, with the capability of placing a large number of biosensor elements on a single substrate (e.g., see U.S. Pat. Nos. 5,200,051 and 5,212,050).
Secondly, because small electrochemical signals can be readily amplified (and subjected to various types of signal processing if desired), electrochemical biosensors have the potential for measuring minute quantities of analyte, and proportionately small changes in analyte levels.
PCT patent application PCT/CA97/00275, published Nov. 6, 1997, publication No. WO 97/41424, discloses a novel electrochemical biosensor having a conductive detection surface, and a hydrocarbon-chain monolayer formed on the surface. Biosensor operation is based on the flow of an ionized redox species across the monolayer, producing a measurable current flow. In one embodiment of the biosensor disclosed, binding of an analyte to its opposite binding member attached to the surface of some of the hydrocarbon chains increases measured current flow by increasing the disorder of the monolayer, making it more permeable to the redox species. In another general embodiment, the opposite binding member is anchored to the monolayer through a coiled-coil heterodimer structure, allowing any selected binding member carried on one .alpha.-helical peptide to be readily attached to a "universal" monolayer surface carrying the opposite .alpha.-helical peptide. This biosensor is capable of detecting and quantifying analyte-binding events and is characterized by: (i) direct electrochemical conversion of the binding event to electrical signal; (ii) a high electron flow "turnover" from each binding event; (iii) adaptable to substantially any analyte, and (iv) good storage characteristics and rapid wetting with sample application.
It would be desirable to adapt this novel biosensor to a wide variety of analytes and to incorporate the biosensor into a card-based device.